Method of charging cellulose digesters



April'l4;' 1959 c. F. ROSENBLAD 2,832,148

' METHOD OF CHARGING CELLULOSE DIGESTERS Filed May 25, 1956 CUR? F," Ross/V5490- INVENTOR.

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ATTORNEY.

United States Patent 2,882,148 METHOD OF CHARGING CELLULOSE DIGESTERS Curt Fredrik Rosenblad, Princeton, NJ. Application May 25, 1956, Serial No. 587,299 Claims priority, application Sweden July 21, 1955 8 Claims. (Cl. 92-7) This invention relates to methods for the cooking of cellulose in a chemical pulping cooking process and is particularly concerned with the charging aspects of such cooking.

It is known to charge a cellulose digester with a digestlo'n liquid of a high temperature in order to achieve rapid impregnation of the chips, and thus to reduce the time for cooking or digestion. The upper limit for the temperature of the digestion liquid in this connection is set by the temperature at which the dissolving of the lignin of the wood sets in under the action of the digestion liquid. In cooking sulphite pulp this takes place at 105- 110" C., and in cooking sulphate pulp at 120140 C. If a good penetration has been achieved it may also be possible to charge the digester with a liquor having a temperature above the end temperature of the cook-up to 160 C. for sulphite and 190 C. for sulphate.

In the method hitherto employed charging is seldom effected at temperatures which exceed 80 for the cooking acid in the case of cooking sulphite pulp, and 90 C. for the cooking liquor in the case of sulphate pulp. This is done in order to avoid evaporation during the charging, which gives SO -degassing With a risk of gypsum being formed in cooking sulphite pulp, and would render simultaneous filling with chips and charging with cooking liquor impossible in cooking sulphate pulp.

The method of the invention renders possible charging of acid and/ or cooking liquor, all hereinafter referred to as cooking liquor, at the respective high limit temperatures mentioned above without gas being set free or evaporation taking place from the charged digestion liquid. The invention is mainly characterized by the digester being filled with a liquid of considerably lower temperature than the said high temperature this liquid being pumped into the digester so that the air and/or gas contained in the digester during that operation will be displaced or forced out of the digester by the liquid, which pressure preferably rises therein. The liquid, hereinafter generally referred to as penetration liquid can consist of cooking liquor of full, or lower than full, concentration, or some other liquid, suitable for such displacement. The said penetration liquid which has beenpumped in is then rapidly forced or displaced through an outlet from the digester by means of hot cooking liquor which is pumped into the digester at the said high temperature, in the course of which, by throttling the said outlet, pressure in the digester is maintained at a height which causes gases dissolved in the hot cooking liquor to remain, at least in the main, dissolved therein in the digester and also have the effect of avoiding evaporation of the hot cooking liquor.

The displacement by means of hot cooking liquor should be carried out so rapidly (for example in 10-15 mins.), that no appreciable displacement takes place during the time necessary for heating up the chips at the top and/ or bottom of the digester.

It is, accordingly, an object of the invention to provide a method for the charging of a digester with hot cooking liquor while preventing gas from being set free or evaporation of the liquor to take place.

Another object is to provide for such charging where the liquor for the charge is heated to the cooking temperature for the particular chips.

Still another object is to provide for speed and economy in thecharging of digesters for the cooking of cellulose.

Further and more detailed objects will be in part pointed out and in part be obvious 'as the description of the invention, taken in conjunction with the accompanying drawing, proceeds.

In that drawing:

Fig. 1 is a diagrammatic showing of one embodiment of a system in accordance with the invention and for performing the method thereof; and

Fig. 2 is a similar view of another embodiment.

According to Fig. 1 the invention is applied as follows 1n practice: A prepared penetration liquid, if necessary preheated to a certain slight extent, is conveyed to a pressure tank 1 through a pipe 18 under control of the valve 18a and, in accordance with the first stage of the invention, is pumped from the tank 1 by means of a pump 2, through a heat exchanger 3 without being heated there, through a pipe 4 and a valve 5 into the lower part of the digester 6. The penetration liquid then penetrates into the digester, and displaces or forces out the air, gas and steam vapour in the digester 6 through a valve 7 at the top of the digester to an exhaust gas-pipe at said top of the digester. Such liquid is pumped in until the digester is full, whereupon valves 5 and 7 are closed, so that the digester stands full of penetration liquid of a relatively low temperature, if desired under pressure which may have been built up by the pump it valve 7 was closed before valve 5.

The second stage of the invention then follows by displacing the penetration liquid already pumped in, by means of cooking liquor of maximum temperature.

On continued pumping of cooking liquor into the digester, this time through a pipe 16 under control of valve 17 to the top of the digester, the penetration liquid already pumped in will be displaced out from the bottom of the digester through a pipe 9, having an overflow valve 8, and via a pipe 21 back to tank 1. The displacing cooking liquor just having been pumped to the digester by way of the heat exchanger 3 is heated by means of heating steam supplied to said heat exchanger from a main pipe-line 20 through a pipe 11 having a valve 10. The quantity of heating steam is controlled automatically by means of a regulator 12 actuated by thermoimpulses carried from pipe 4 through the impulse line 12a so that the newly supplied cooking liquor is steadily heated up to the desired maximum temperature. Overflow valve 8 is set to open at the pressure in the digester, which is required to prevent steam and gas from being liberated or set free there from the hot cooking liquor, which thus will be charged under this displacement pressure without giving oil gas or steam. On completion of the process, which can be read off on the thermometer 13,-

valve 17 is closed and the cooking or digestion can start in the usual manner. Part 14 is a drainage sieve, and 15 is a discharge outlet for condensate.

According to Fig. 2 a cold penetration liquid other than the prepared cooking liquor is used for the first, or gas expulsion, stage. Examples are a liquid of lower concentration than prepared cooking liquor, or water, stored in a special tank 1a, which can be filled through a valve 16. In this case, tank 1 contains, as in Fig. 1, prepared cooking liquor heated to th e highest temperature permitted. Continuous heating is obtained by pumping this liquor in circulation by means of a pump 22 through a heat exchanger 3a, which receives heating steam from the main pipe 20 through pipe line 11 having valve 10 therein. In the first stage the digester 6 is filled with cold liquid from tank 1a through a pump 2a, a pipe 4a and valve 5 with the same effect and with the same conclusion of the stage as in Fig. 1. In the second stage, cold liquid is displaced from the digester, as in Fig. 1, but it goes back to tank 1a to which pipe-line. 21 leads in this case, the displacement being effected by means of hot cooking liquor which is pumped into the digester from tank 1, in this case without being heated during the course of the pumping, since this liquid is obtained from this tank in heated condition.

The advantage of this latter arrangement lies especially in the fact that the prepared cooking liquor is exposed to the least possible loss of gas, intermixing of acid and dilution as it is not employed for the first stage. The heating of the cooking liquor is continuous and thus does not cause great load peaks in the steam system. Only one digester is shown in the figures, but several digesters can of course be connected up in an identical manner between the pipes 4, 19, 20 and 21. By a suitable phase displacement between the cooking processes, one and the same tank and heating system may be utilized for all digesters. The costs of this system Will in this way be relatively moderate for a large digester plant, and even a heat exchanger such as that shown at 3 can be made to work fairly continuously in this way.

With the guidance of the illustrated embodiments, which serve only as examples, numerous other modifications and combinations can of course be made within the scope of the invention. In Fig. 2, for example, the heat exchanger 311 can be substituted for heat exchanger 3 as in Fig. l, or vice versa. Furthermore, instead of the indirect heating of the cooking liquor by means of heat exchanger 3a direct heating may be used by means of steam which is led into tank 1.

In order to improve the penetration, the system can in all cases be combined in the first stage with the quick penetration method which is known in itself and comprises filling the digester to the top with liquid and then exposing this liquid to repeated shocklike variations of pressure so that air and gas will be forced to leave the cells of the wood and other cavities in an especially effective manner, thus enabling the liquid to penetrate into them.

If one can be satisfied without the advantage of displacement in a direction from the top downwards in the second stage, the pipe system can be simplified by arranging pipe 9 with overflow valve 8 to emerge from the top of the digester. In this case the pipe 4 and the valve 5 can be employed for supplying the hot liquor during this stage, whereupon pipe 16 with the valve 17 are then When, according to Fig. 2, for example, other liquidthan cooking liquor of full concentration is pumped in first it may be to advantage to use a liquid containing certain chemicals particularly suitable for the penetration of the chips prior to being cooked. Such chemicals as have been consumed by the liquid during the penetration should then be made good, for example by being.

added to tank 1a or some other part of the circuit so that the same liquid can be used repeatedly.

Since certain changes may be made in carrying out the above method without departing from the scope of the invention, it is intended that all matter contained inv the above description shall be interpreted as illustrative and not in a limiting sense.

Having described my invention what I claim as new and desire to secure by Letters Patent is:

1. The method of charging a cellulose digester with.

cooking liquor heated to substantially cooking temperature which comprises, charging a digester with cellulosic material to be cooked, thereafter filling the digester with a liquid of substantially lower temperature than the eventual temperature desiredto displace gas and air from the digester, rapidly displacing said liquid with hot cooking liquor, throttling the outlet of said displaced liquid, and providing pressure in said digester to maintain the gasses dissolved in said hot cooking liquor in solution therein and to avoid evaporation of said hot cooking liquor- 2. The method of charging a cellulose digester with cooking liquor heated to substantially cooking temperaturev which comprises, charging a digester with cellulosic material to be cooked, thereafter filling the digester with a liquid of substantially lower temperature than the eventual temperature desired, rapidly displacing said liquid with cooking liquor and heating said cooking liquor indirectly as it is pumped to said digester, throttling the outlet of said displaced liquid and providing pressure in said digester to maintain the gasses dissolved in said hot cooking liquor in solution therein and to avoid evaporation of said hot cooking liquor.

3. The method of charging a cellulose digester with cooking liquor heated to substantially cooking temperature which comprises, charging a digester with cellulosic material to be cooked, thereafter filling the digester with a liquid of substantially lower temperature than the even tual temperature desired, continuously heating a supply of cooking liquor adequate to replace said liquid, rapidly displacing said liquid with said heated cooking liquor, throttling the outlet of said displaced liquid and providing pressure in said digester to maintain the gasses dissolved in said hot cooking liquor in solution therein and to avoid evaporation of said hot cooking liquor.

4. The method of charging a cellulose digester with cooking liquor heated to substantially cooking temperature which comprises, charging a digester with cellulosic material to be coked, thereafter drawing liquid from a source and of substantially lower temperature than the eventual temperature desired, filling the digester with said liquid to displace gas and air from the digester, rapidly displacing said liquid with hot cooking liquor, throttling the outlet of said displaced liquid, returning said displaced liquid to said source and providing pressure in said digester to maintain the gasses dissolved in said hot cooking liquor in solution therein and to avoid evaporation of said hot cooking liquor.

5. The method of charging a cellulose digester with cooking liquor heated to substantially cooking temperature which comprises, charging a digester with cellulosic material to be cooked, thereafter filling the digester with a liquid of substantially lower temperature than the eventual temperature desired, but of full concentration for cooking purposes to displace gas and air from the digester, rapidly displacing said liquid with hot cooking liquor and forcing said liquid into a separate high pressure tank in which the same is heated continuously to be ready for use in another charging stage, throttling the outlet of said displaced liquid from said digester and providing pressure in said digester to maintain the gasses dissolved in said hot cooking liquor in solution therein and to avoid evaporation of said hot cooking liquor.

6. The method as in claim 1 and including, collecting said displaced liquid under low pressure in a separate container ready for re-employment as the first liquid to beused in a subsequent charging of a digester.

7. The method as in claim 1 and including, adding suitable chemicals to said first liquid to facilitate penetration of the cellulosic material in the digester.

8. The method of charging a cellulose digester with cooking liquor heated to substantially cooking temperature which comprises, charging a digester with cellulosic material and thereafter filling the digester with a liquid of substantially lower temperature than the eventual temperature desired, said liquid being introduced under sufiicient pressure to displace gas and air from the digester, rapidly displacing said liquid with hot cooking liquor under a higher pressure, throttling the outlet of said liquid to maintain said higher pressure in said digester and thereby maintaining the gasses dissolved in said but cooking liquor in solution therein while avoiding evaporation of said hot cooking liquor.

Maste Aug. 9, 1892 Kreissler Jan. 1, 1935 

1. THE METHOD OF CHARGING A CELLULOSE DIGESTER WITH COOKING LIQUOR HEATED TO SUBSTANTIALLY COOKING TEMPERATURE WHICH COMPRISES, CHARGING A DIGESTER WITH MATERIAL TO BE COOKED, THEREAFTER FILLING THE DIGESTER WITH A LIQUID OF SUBSTANTIALLY LOWER TEMPERATURE THAN THE EVENTUAL TEMPERATURE DISIRED TO DISPLACED GAS AND AIR FROM THE DIGESTER, RAPIDLY DISPLACING SAID LIQUID WITH HOT COOKING LIQUOR, THROTTLING THE OUTLET OF SAID DISPLACED LIQUID, AND PROVIDING PRESSURE IN SAID DIGESTER TO MAINTAIN THE GASSES DISSOLVED IN SAID HOT COOKING LIQUOR IN SOLUTION THEREIN AND TO AVOID EVAPORATION OF SAID HOT COOKING LIQUOR. 